1. Field of the Invention
The present invention is generally directed to the field of biometric imaging. In particular, the present invention is directed to a method for calibrating and correcting settings in a fingerprint scanner.
2. Related Art
Biometrics is a science involving the analysis of biological characteristics. Biometric imaging captures a measurable characteristic of a human being for identity purposes. See, e.g., Gary Roethenbaugh, Biometrics Explained, International Computer Security Association, Inc., pp. 1-34, (1998), which is incorporated herein by reference in its entirety.
One type of biometric imaging system is an Automatic Fingerprint Identification System (AFIS). Automatic Fingerprint Identification Systems are used for law enforcement purposes. Law enforcement personnel collect fingerprint images from criminal suspects when they are arrested. Law enforcement personnel also collect fingerprint images from crime scenes. These are known as latent prints.
Tenprint scanners are a common type of AFIS system. Tenprint scanners produce forensic-quality tenprint records of rolled and plain impression fingerprint images. Tenprint scanners must be sufficiently reliable to meet rigid image standards, such as NIST image requirements. Normal usage of the tenprint scanner over time, as well as variations in temperature, dirt and dust, etc., cause the performance level of the tenprint scanner to drift with respect to certain optimal settings. Settings needing periodic adjustment and correction include brightness, contrast, focus, and geometric distortion. What is needed is a system and method that periodically calibrates the tenprint scanner to maintain optimal settings. What is also needed is a system and method of calibration and correction that provides increased tolerances in the optical design of the tenprint scanner.
The present invention solves the above-mentioned needs by providing a system and method for performing calibration and correction of optimal settings in a fingerprint scanner. Briefly stated, the present invention is directed to a calibration and correction procedure for a fingerprint scanner. The calibration and correction procedure performs an automatic calibration (auto-calibration) procedure and a gray level linearity procedure. The auto-calibration procedure includes a brightness function to correct for distortions in brightness, a focus check function to identify when the fingerprint scanner is out of focus, and a geometric distortion function to correct for imperfect linearity in the geometry of the fingerprint scanner. The gray level linearity procedure corrects for linear distortions in brightness and contrast of gray levels.
The present invention performs the auto-calibration of the fingerprint scanner on a periodic basis. Calibration of the fingerprint scanner may also be performed at the request of an operator as well. Automatic calibration on a frequent basis, such as a daily basis, provides increased tolerances in the optical design of the fingerprint scanner.
The gray level linearity calibration and correction procedure is performed at the factory and/or by field service technicians. In another embodiment of the present invention, the gray level linearity calibration and correction procedure is performed by an operator in a manner similar to the auto-calibration procedure.
Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.